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127 lines
6.3 KiB
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<title>Ifdef - The C Preprocessor</title>
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<a name="Ifdef"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="If.html#If">If</a>,
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Up: <a rel="up" accesskey="u" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>
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</div>
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<h4 class="subsection">4.2.1 Ifdef</h4>
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<p><a name="index-g_t_0023ifdef-85"></a><a name="index-g_t_0023endif-86"></a>
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The simplest sort of conditional is
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<pre class="smallexample"> #ifdef <var>MACRO</var>
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<var>controlled text</var>
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#endif /* <var>MACRO</var> */
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</pre>
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<p><a name="index-conditional-group-87"></a>This block is called a <dfn>conditional group</dfn>. <var>controlled text</var>
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will be included in the output of the preprocessor if and only if
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<var>MACRO</var> is defined. We say that the conditional <dfn>succeeds</dfn> if
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<var>MACRO</var> is defined, <dfn>fails</dfn> if it is not.
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<p>The <var>controlled text</var> inside of a conditional can include
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preprocessing directives. They are executed only if the conditional
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succeeds. You can nest conditional groups inside other conditional
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groups, but they must be completely nested. In other words,
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‘<samp><span class="samp">#endif</span></samp>’ always matches the nearest ‘<samp><span class="samp">#ifdef</span></samp>’ (or
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‘<samp><span class="samp">#ifndef</span></samp>’, or ‘<samp><span class="samp">#if</span></samp>’). Also, you cannot start a conditional
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group in one file and end it in another.
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<p>Even if a conditional fails, the <var>controlled text</var> inside it is
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still run through initial transformations and tokenization. Therefore,
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it must all be lexically valid C. Normally the only way this matters is
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that all comments and string literals inside a failing conditional group
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must still be properly ended.
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<p>The comment following the ‘<samp><span class="samp">#endif</span></samp>’ is not required, but it is a
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good practice if there is a lot of <var>controlled text</var>, because it
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helps people match the ‘<samp><span class="samp">#endif</span></samp>’ to the corresponding ‘<samp><span class="samp">#ifdef</span></samp>’.
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Older programs sometimes put <var>MACRO</var> directly after the
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‘<samp><span class="samp">#endif</span></samp>’ without enclosing it in a comment. This is invalid code
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according to the C standard. CPP accepts it with a warning. It
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never affects which ‘<samp><span class="samp">#ifndef</span></samp>’ the ‘<samp><span class="samp">#endif</span></samp>’ matches.
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<p><a name="index-g_t_0023ifndef-88"></a>Sometimes you wish to use some code if a macro is <em>not</em> defined.
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You can do this by writing ‘<samp><span class="samp">#ifndef</span></samp>’ instead of ‘<samp><span class="samp">#ifdef</span></samp>’.
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One common use of ‘<samp><span class="samp">#ifndef</span></samp>’ is to include code only the first
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time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>.
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<p>Macro definitions can vary between compilations for several reasons.
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Here are some samples.
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<ul>
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<li>Some macros are predefined on each kind of machine
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(see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide
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code specially tuned for a particular machine.
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<li>System header files define more macros, associated with the features
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they implement. You can test these macros with conditionals to avoid
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using a system feature on a machine where it is not implemented.
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<li>Macros can be defined or undefined with the <samp><span class="option">-D</span></samp> and <samp><span class="option">-U</span></samp>
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command-line options when you compile the program. You can arrange to
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compile the same source file into two different programs by choosing a
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macro name to specify which program you want, writing conditionals to
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test whether or how this macro is defined, and then controlling the
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state of the macro with command-line options, perhaps set in the
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Makefile. See <a href="Invocation.html#Invocation">Invocation</a>.
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<li>Your program might have a special header file (often called
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<samp><span class="file">config.h</span></samp>) that is adjusted when the program is compiled. It can
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define or not define macros depending on the features of the system and
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the desired capabilities of the program. The adjustment can be
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automated by a tool such as <samp><span class="command">autoconf</span></samp>, or done by hand.
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</ul>
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</body></html>
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